Slip critical bolts_Steel Structures 2

Yes. The only consequence is the additional movement at the joint that may or may not affect the performance of the structure.

Agree with structSU10, and sometimes the reverse is practical, too (change a bearing design to slip critical). I had one project with heavily loaded connections where the fabricator asked if the connections needed to be slip critical. After thinking about it, said yes. Only change needed was the fabricator's proper preparation of the faying surfaces.

Not necessarily. It most cases, you probably wouldn't get a collapse, but I wouldn't say it will "work" as a bearing type.

If the bolts are in the flanges of a beam at a moment connection, the additional movement could increase the moment at the beam's mid-span, which could cause a serviceability failure (excessive deflection causing floor finish cracking or alignment problems) or a strength problem if the design is tight enough.

If it's in a brace connection, the additional slip would allow the frame to deflect laterally, increasing P-Delta effects. Probably not a big deal on low rise, but a tall mid-rise or high rise it could cause serious problems to the system as a whole.

Then, there's no guarantee that the bolt configuration or cross sectional area will perform as a bearing connection. To be honest, I've never checked, so maybe there is a coincidental relationship that makes it work every time, but I wouldn't count it.

If the added deflections/slip are not a problem, and the system works with a partially restrained moment connection rather than a fully restrained one, and you've checked the connections in bearing to be sure they work - then sure, they'll work if friction fails. But then, if you're going to go through all that, why not just do a bearing type connection in the first place and save the money for installation and inspections of slip critical connections?

Snug tight are generally slightly stronger than slip critical so there is generally no issue with the exception of connections that see a lot of stress reversal.

Dik

There is a table in the bolt spec which indicates the different bolt pretensioning requirements and conditions. Loss or lack of pretensioning might violate the spec for certain conditions (such as bolts in direct tension).

Lack of pretension would mean certain pre-qualified seismic joints would not be qualified and might not act as designed and intended in a seismic event.

Don't know about AISC but a direct quote from AASHTO that might be of use to you:

"As loading is increased to a level in excess of the frictional resistance between the faying surfaces, slip occurs, but failure in the sense of rupture does not occur. As a result, slip-critical connections are able to resist even greater loads by shear and bearing against the connected material. The strength of the connection is not related to the slip load. These specifications require that the slip resistance and the shear and bearing resistance be computed separately."

AASHTO typically prevents slip under the Service Level Conditions and Bearing/Shear under Strength Level conditions.

One thing to consider is if you are using oversize holes, short slotted holes, or long slotted holes loaded in the direction of the slot. You may see a considerable amount of movement after slip before you engage the "bearing" of the bolt. There are some conditions where I could see that this would not be acceptable.

Do the calculation yourself and find out. This is a very basic question and you should probably learn the background. Hint: read the first sentence of AISC Section J3.8.

If the SC bolts are in oversized holes or in slots with the force vector other than perpendicular to the slot (less than 80 degrees, more than 100 degrees), you will have a problem. Otherwise if the bolts are in standard holes and they slip you probably won't have a collapse danger, but you may have more deformation than anticipated.

Yes, guys, I too agree, this condition would not apply for the oversized holes and slotted holes along force direction, my concern is "will this work" for standard holes? If a slip critical bolt loses its pretension slightly along with the duration of structure, is it possible?

Thanks in advance!!

If a slip critical bolt loses its pretension slightly along with the duration of structure, is it possible?

Click to expand...

They better not, or there's a huge number of structures out there that will develop deflections in places they shouldn't. Sure, you could get a defective bolt - anything is possible, but it's not supposed to happen, and it's not typically considered in design.

Also, the design friction values for the faying surfaces in various condition are typically fairly conservative.

Rod Smith, P.E., The artist formerly known as HotRod10

In this condition, What would happen if faying surface is painted?

Thanks in advance!!

Well the ultimate limit state of the connection is always the bearing state without friction
Slip critical connections are better for lower deformations...but at the end of the line the load capacity is independent of friction
That's how we design connections ....

If it's painted, it's not a slip critical connection. At least not by AISC standards.

There are coating systems out there that have been tested and approved to meet RCSC and AISC standards for both type A and B surface preparation, so you can paint the faying surface as long as you use the correct type of paint.

sbisteel - good to know. Thanks for that info.

Guys, why there is no chance will it work as bearing type? as the slip critical bolts are pre-tensioned load will be transferred through shearing and bearing type bolts the load will be transferred through-bolt bearing. Am I wrong?

Thanks in advance!!

What do you mean when you say work? A slip critical connection is designed to NOT SLIP. Therefore, I would say that a slip critical joint that has slipped and is now bearing does not work - because it slipped. As others have mentioned, some codes and standards allow you to look at service load levels under no slip, but the higher ultimate loading conditions can be allowed to slip and go into bearing. To do this, you'd have to look at the added deflection and ensure the structure is still stable under ultimate loading with the higher deflections. But then, most seismic requirements do not allow this. So it's going to depend on the kind of structure, where it's built, and what the design requirements for the connection are.

Once the slip-critical state has been lost, it it going to be in a "bearing" condition, regardless of design intent. In order to prevent total connection failure/rupture, every slip-critical connection should be checked for any possible bearing limit states. Like someone above said, it might affect your deflection criterion and transfer additional loads to other components of your structural system, but it should be a functional connection.

"In this condition, What would happen if faying surface is painted?"

There are only a couple of primers that I'm aware of the can coat the faying surface... and no topcoats. You don't want to coat the slip critical surfaces.

Dik